Using normal euploid cells in cultrue, we propose to continue research pertinent to X inactivation and control of cell phenotype. We will determine if the entire X is subject to inactivation by analyzing clones from heterozygotes for X-linked variants. We will continue our attempts to isolate X specific DNA from human cells. We will continue studies of X chromosome phenotype in triploid cells. In search of genetical heterogeneity, we will carry out complementation analysis in in cell hybrids derived from phenotypically similar inborn errors of metabolism. We will continue to develop the D-valine selective system for epithelial cells which have D-amino acid oxidase, an enzyme present only in differentiated cells.